Systems and methods for narrowing a wavelength emission of light

ABSTRACT

The present invention generally relates to methods and systems for narrowing a wavelength emission of light. In certain aspects, methods of the invention involve transmitting light through a filter and passing a portion of the filtered light through a gain chip assembly at least two times before that portion of light passes again through the filter.

RELATED APPLICATION

This application claims the benefit of and priority to U.S. ProvisionalApplication Ser. No. 61/745,270, filed Dec. 21, 2012, which isincorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to methods and systems fornarrowing a wavelength emission of light.

BACKGROUND

Optical coherence tomography (OCT) is an established medical imagingtechnique that relies on light for producing an image. In OCT, lightfrom a broadband light source is split by an optical fiber splitter withone fiber directing light to a sample path and the other fiber directinglight to a reference path mirror. An end of the sample path is typicallyconnected to a scanning device. The light reflected from the scanningdevice is recombined with the signal from the reference mirror to forminterference fringes, which are transformed into a depth resolved image.

Numerous different OCT techniques have been developed, including sweptsource OCT. In swept source OCT, a narrowband light source is rapidlytuned over a broad optical bandwidth, and spectral components areencoded in time. Thus, image quality in swept source OCT relies on theswept laser source achieving very narrow bandwidths at very highfrequencies (e.g., 20-200 kHz) over a very short period of time (e.g.,10,000-10,000,000 Sweeps/sec).

A typical set-up for a swept source OCT system uses a ring resonatorthat includes an optical amplifier, a tunable filter, and an opticalcoupler. Laser light at a specific bandwidth is produced by the opticalamplifier and sent through the filter. The filter assists in maintainingthe outputted light at the specific bandwidth. From the filter, thelight travels to the optical coupler where a portion of the light isdirected to an interferometer and the remainder of the light returns tothe optical amplifier. The process repeats to achieve differentbandwidths of light.

A problem with this set-up is that a single pass through an opticalamplifier cannot impart enough energy to the light to obtain the verynarrow bandwidths in an optimal amount of time. Another problem withthis set-up is that a single pass through an optical amplifier cannotimpart enough energy to the light to prevent broadening of the obtainedbandwidth, particularly at higher frequencies. Both problems lead todegradation of image quality.

SUMMARY

The invention provides a ring resonator set-up for optical coherencetomography (OCT) in which light passes through an optical amplifiertwice before it proceeds to an OCT interferometer. As encompassed by theinvention, a first pass of light exiting the optical amplifier isdirected back through the amplifier rather than continuing through thering resonator. Light exiting the amplifier can be sent back through theamplifier by using, for example, a mirror. The light re-enters theamplifier, wherein it is amplified a second time. This re-amplifiedlight is then transmitted through the ring resonator where it cancontinue onto the OCT interferometer. An optical coupler may be used tocoordinate the passage of light between the amplifier and the rest ofthe ring resonator. The provided ring resonators may also include afilter for maintaining the narrow bandwidth of light after it has beenamplified twice by the optical amplifier.

By passing the light through the amplifier twice during a single tripthrough the resonator, a narrowed bandwidth of light is achieved fasterthan only passing the light through an interferometer once beforeproceeding to the interferometer. The second pass through the amplifierin a single trip also imparts additional energy to the light, whichprevents undesired broadening of the obtained bandwidth at higherfrequencies. Accordingly, the provided ring resonators are able toproduce high quality OCT images that avoid the image degradationproblems associated with prior art ring resonator configurations.

The provided ring resonator can serve as a light source for a variety ofapplications, including medical imaging. For example, light leaving thering resonator can be directed towards an OCT system. As noted above,the present invention is particularly suited for OCT due to theimprovement of image quality.

In addition to the provided resonators, the invention also encompassesmethods for narrowing a wavelength of light. The methods involve passinglight through an optical amplifier of a ring resonator twice during onepass around the ring resonator. Using the provided methods, a narrowbandwidth of light is achieved in an optimal amount of time and imagequality is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a gain curve for a booster optical amplifier.

FIG. 2 is a diagram of a laser.

FIG. 3 schematically depicts a system of the invention, according tocertain embodiments.

FIG. 4 is a schematic of an optical coherence tomography (OCT) systemfor use with systems and methods of the invention.

FIG. 5 is a schematic diagram of the imaging engine of an OCT system.

FIG. 6 is a diagram of a light path in an OCT system.

FIG. 7 shows the organization of a patient interface module in an OCTsystem.

DETAILED DESCRIPTION

The invention generally relates to methods and systems for producingnarrow emissions of light. The invention can involve transmitting lightthrough a filter and passing a portion of the filtered light through again chip assembly at least two times before that portion of thefiltered light passes again through the filter. Broadband light isemitted from a light source and transmitted through a filter. Thefiltered light is passed through a gain chip assembly in which the lightsignal is amplified. The amplified light leaves the gain chip assembly,whereupon a portion of that light is sent back into the gain chipassembly, where it is amplified a second time. This re-amplified lightis then directed back through the filter, wherein the whole process canrepeat itself. Accordingly, the re-amplification of filtered light priorto its return to the filter can include sending the light through a gainmedium in the gain chip assembly back through the gain chip assembly.This results in a second pass through the assembly gain medium beforepassing again through the filter.

Any optical filter is useful for practicing the invention. Asencompassed by the invention, the filter receives light from a broadbandlight source and emits light of a predetermined wavelength. An opticalfilter typically has a peak reflectivity and a background reflectivity.The peak reflectivity indicates an amount of light output (reflected) atthe specified wavelength, wherein a desired wavelength can be set in atunable filter by placing mirrors in an etalon at an appropriatedistance apart. The background reflectivity indicates an amount of lightoutput at wavelengths other than the desired wavelength.

Typical filters might have, for example, a 20% peak reflectivity and a0.02% background reflectivity. The ratio of these numbers (10³) definesthe filter contrast ratio, expressed in decibels (dB) (here, 30 dB).Thus, if light of a certain wavelength, for example, 1200 nm, isintended, the filter will transmit light at 1200 nm as well as a broadspectrum of light at lower power in a ratio of 30 dB.

In some embodiments, systems of the invention include an optical filterthat can be tuned to a desired wavelength, i.e., a tunable filter.Amplified light of a selected wavelength is obtained by tuning thefilter to that wavelength and sending the light into the gain mediumwith sufficient input power to achieve a desired output power.

Where an optical system requires a particular wavelength of amplifiedlight, the light source may include an optical filter module such as atunable optical filter in optical communication with a gain component.

In certain embodiments of the invention, the tunable filter is avoltage-controlled optical attenuator. In a VCOA, an optical attenuatoris placed between an input and output lens for obstructing the path ofan incoming light beam. The attenuator has variable attenuation(reflection, absorption, etc.) which is controlled for maintaining apreset power or wavelength of the outgoing light beam. To this end, afraction of the outgoing signal is diverted to an output detector byreflecting off an end face of a lens, and processed for obtaining acontrol signal representative of the output power. An electric outputdisplaces the attenuator to a position corresponding to a preset outputpower. Further detail on VCOAs can be found, for example, in U.S. Pat.No. 5,745,634, incorporated by reference herein in its entirety.

The invention also encompasses the use of a gain chip assembly thatamplifies the power of light that is transmitted through it. As providedby the invention, light travels from the filter to the gain chipassembly for amplification. The gain chip assembly, or gain component,generally refers to any device known in the art capable of amplifyinglight such as an optical amplifier, laser, or any component employing again medium. A gain medium is a material that increases the power oflight transmitted through the gain medium. Exemplary gain mediumsinclude crystals (e.g., sapphire), doped crystals (e.g., yttriumaluminum garnet, yttrium orthovanadate), glasses such as silicate orphosphate glasses, gasses (e.g., mixtures of helium and neon, nitrogen,argon, or carbon monoxide), semiconductors (e.g., gallium arsenide,indium gallium arsenide), and liquids (e.g., rhodamine, fluorescein).

When light interacts with the material of the gain medium, severaloutcomes may be obtained. Light may be transmitted through the materialunaffected or reflect off a surface of the material. Alternatively, anincident photon of light can exchange energy with an electron of an atomwithin the material by either absorption or stimulated emission. If thephoton is absorbed, the electron transitions from an initial energylevel to a higher energy level. In three-level system, there is atransient energy state associated with a third energy level.

When electron returns to ground state, a photon is emitted. When photonsare emitted, there is a net increase in power of light within the gainmedium. In stimulated emission, an electron emits energy ΔE through thecreation of a photon of frequency v₁₂ and coherent with the incidentphoton. Two photons are coherent if they have the same phase, frequency,polarization, and direction of travel. Equation 1 gives the relationshipbetween energy change ΔE and frequency v₁₂:ΔE=hv ₁₂  (1)where h is Plank's constant. Light produced this way can be temporallycoherent, i.e., having a single location that exhibits clean sinusoidaloscillations over time.

An electron can also release a photon by spontaneous emission. Amplifiedspontaneous emission (ASA) in a gain medium produces spatially coherentlight, e.g., having a fixed phase relationship across the profile of alight beam.

Emission prevails over absorption when light is transmitted through amaterial having more excited electrons than ground state electrons—astate known as a population inversion. A population inversion can beobtained by pumping in energy (e.g., current or light) from outside.Where emission prevails, the material exhibits a gain G defined byEquation 2:G=10 Log₁₀(P _(out) /P _(in)) dB  (2)where P_(out) and P_(in) are the optical output and input power of thegain medium.

As encompassed by the invention, the gain chip component can be anoptical amplifier or a laser. An optical amplifier is a device thatamplifies an optical signal directly, without the need to first convertit to an electrical signal. An optical amplifier generally includes again medium (e.g., without an optical cavity), or one in which feedbackfrom the cavity is suppressed. Exemplary optical amplifiers includedoped fibers, bulk lasers, semiconductor optical amplifiers (SOAs) andRaman optical amplifiers. In doped fiber amplifiers and bulk lasers,stimulated emission in the amplifier's gain medium causes amplificationof incoming light. In SOAs, electron-hole recombination occurs. In Ramanamplifiers, Raman scattering of incoming light with phonons (i.e.,excited state quasiparticles) in the lattice of the gain medium producesphotons coherent with the incoming photons.

Doped fiber amplifiers (DFAs) are optical amplifiers that use a dopedoptical fiber as a gain medium to amplify an optical signal. In a DFA,the signal to be amplified and a pump laser are multiplexed into thedoped fiber, and the signal is amplified through interation with thedoping ions. The most common DFA is Erbium Doped Fiber Amplifier (EDFA),which features a silica fiber with an Erbium-ion doped core. Anexemplary EDFA is the Cisco ONS 15501 EDFA from Cisco Systems, Inc. (SanJose, Calif.)

Semiconductor optical amplifiers (SOAs) are amplifiers that use asemiconductor to provide the gain medium. In a SOA, input light istransmitted through the gain medium and amplified output light isproduced. An SOA includes n-cladding layer and p-cladding layer. The SOAalso typically includes a group III-V compound semiconductor such asGaAs/AlGaAs, InP/InGas, InP/InGaAsP and InP/InAlGaAs, though anysuitable semiconductor material may be used.

A typical semiconductor optical amplifier includes a doubleheterostructure material with n-type and p-type high band gapsemiconductor layers around a low band gap semiconductor. The high bandgap layers are sometimes referred to as p-cladding layers (which havemore holes than electrons) and n-cladding layers (which have moreelectrons than holes). The carriers are injected into the gain mediumwhere they recombine to produce photons by both spontaneous andstimulated emission. The cladding layers also function as waveguides toguide the propagation of the light signal. Semiconductor opticalamplifiers are described in Dutta and Wang, Semiconductor OpticalAmplifiers, World Scientific Publishing Co. Pte. Ltd., Hackensack, N.J.(2006) the contents of which are hereby incorporated by reference intheir entirety.

Booster Optical Amplifiers (BOAs) are single-pass, traveling-waveamplifiers that only amplify one state of polarization generally usedfor applications where the input polarization of the light is known.Since a BOA is polarization sensitive, it can provide desirable gain,noise, bandwidth, and saturation power specification. In someembodiments, a BOA includes a semiconductor gain medium. The input andoutput of BOA can be coupled to one or more waveguides on an opticalamplifier chip. FIG. 1 is a gain curve for a COTS booster opticalamplifier.

Optical amplifier components can be provided in a standard 14-pinbutterfly package with either single mode fiber (SMF) or polarizationmaintain fiber (PMF) pigtails, which can be terminated with any fixedconnection (FC) connector such as an angled physical connection (FC/APC)connector. Optional polarization-maintaining isolators can be providedat the input, output, or both. In certain embodiments, the gain chipassembly includes a wavelength dependent reflector as a reflectivesurface of the optical amplifier, such as a mirror or one or more facetsof the gain medium.

A laser generally is an optical amplifier in which the gain medium ispositioned within an optical resonator (i.e., an optical cavity) asdiagrammed in FIG. 2. In certain embodiments, an optical resonator is anarrangement of mirrors that forms a standing wave cavity resonator forlight waves, e.g., a pair of mirrors on opposite ends of the gain mediumand facing each other. The pair of mirrors includes a high reflector 217and output coupler 205 surrounding the gain medium 201. Incident light221 reflects between the mirrors creating standing wave 213. Some lightis emitted as a laser beam 209. Where laser light is desired, the gainmedium is positioned in an optical cavity. The optical cavity confineslight in the gain medium, thereby feeding it amplified light backthrough the gain medium allowing it to be amplified again. Input lightresonates between the mirrors while being re-amplified by the gainmedium until the lasing threshold is surpassed and laser light isproduced. This results in a positive feedback cycle tending to increasethe gain G of the optical amplifier.

In a laser, one of the mirrors of the optical cavity is generally knownas the high reflector while the other is the output coupler. Typically,the output coupler is partially transparent and emits the output laserbeam. In certain embodiments, the invention provides a wavelengthdependent reflector as a reflective surface with laser, such as one ofthe mirrors (e.g., the output coupler) or one of the facets of the gainmedium.

A laser can be provided, for example, as a COTS component in a 14-pinbutterfly package with either SMF or PMF pigtails. One such exemplarylaser is the PowerPure 1998 PLM, a 980 nm pump laser module with Bragggrating available from Avanex Corporation (Fremont, Calif.).

In certain embodiments, a gain component such as an optical amplifier ora laser amplifies light in a frequency-specific manner. A gain componentincludes a gain medium having a gain coefficient g (gain per unitlength) that is a function of the optical frequency of the incidentsignal w. The gain coefficient at a given frequency g(ω) is given byequation 3:g(ω)=g ₀/(1+(ω−ω₀)² T ² +P/Ps)  (3)where g₀ is the peak gain of the medium, P is the optical power of thesignal being amplified, Ps is the saturation power of the gain medium,ω₀ is an atomic transition frequency of the medium, and T is a dipolerelaxation time. Where incident light has a frequency ω, a gain mediumhas a gain coefficient g(ω) and gain is given by Equation 4:G(ω)=exp[g(ω)L]  (4)where L is a length of the gain medium.

The power of amplified light at a distance z from the input end of again medium is given by Equation 5:P(z)=P _(in)exp(gz)  (5)

Gain coefficient g has an inverse square relationship to (ω−ω₀) (seeEquation 3) and power P(z) is exponentially related to gain coefficientg. Thus, the gain of a gain medium is higher for optical frequencies wcloser to ω₀. FIG. 8 shows gain as a function of wavelength for atypical gain medium. As shown by the peak of the gain curve, the gainmedium has a “peak gain.”

If light of various wavelengths is amplified by the medium (at powerswell below the saturation power Ps of the gain medium), light having awavelength at or near the peak gain will be amplified to a greaterdegree than light having a wavelength not at or near the peak gain.

For any wavelength of light, if the gain is greater than the loss,lasing can result in which the light is emitted as a laser beam. Theconditions at which gain equals loss is the lasing threshold for afrequency of light. The lasing threshold is lowest at the peak gain andlight having a wavelength at the peak gain is more readily and morerobustly amplified than at other wavelengths. Consequently, the gainmedium most readily lases light at the peak gain.

Unintentional lasing is known as parasitic lasing. If light transmittedthrough the medium has sufficient power, wavelengths near the peak gainwill cross the lasing threshold, causing lasing. This parasitic lasingleaches power from the system, reduces coherence length of signal light,and introduces noise into the signal. Due to the shape of the gain curvein a typical gain medium, parasitic lasing is problematic near the peakgain.

Devices and methods of the invention mitigate parasitic lasing andimprove image quality. In one embodiment, systems and methods of theinvention mitigate parasitic lasing by optimizing the time required toreach amplified light of a desired wavelength. Accordingly, undesiredwavelengths have less opportunity to become amplified and also surpassthe lasing threshold. In addition, image quality is also improved byreducing the time necessary to reach a desired wavelength.

In certain aspects, the gain component produces new infrared light fromincident light delivered by a filter module in optical connection to thegain component. Preferably, the reflector is an output coupler and thegain component is a semiconductor optical amplifier. Systems of theinvention further include any other compatible component known in theart. Exemplary components include interferometers, couplers/splitters,controllers, and any other device known in the art. Systems of theinvention may include input and output mechanisms, such as an outputmechanism configured to be coupled to a fiber optic interferometer orother imaging device. An optical system may include a controllercomponent. For example, systems can include the LDC1300 butterfly LD/TECcontroller from Thorlabs (Newton, N.J.). The LD/TEC controller and mountallows a system to be controlled by a computer. In certain embodiments,optical systems are integrated into an optical networking platform suchas the Cisco ONS 15500 Dense Wave Division Multiplexer.

In certain embodiments, the system includes an interferometer such as afiber optic interferometer. An interferometer, generally, is aninstrument used to interfere waves and measure the interference.Interferometry includes extracting information from superimposed,interfering waves.

As encompassed by the invention, a portion of the amplified lightleaving the gain chip assembly is directed back into the assembly. Incertain aspects, this re-direction is accomplished using a partialmirror. Mirrors typically reflect uniformly over a broad spectral range.In contrast, partial mirrors work at off-normal angles of incidence,thereby reflecting only a portion of the light while transmitting theremainder. In certain aspects, a partial mirror is appropriatelypositioned outside the gain chip assembly so that when light leaves theassembly, it is reflected by the partial mirror back into the assembly.Accordingly, light is amplified a second time upon re-entering the gainchip assembly.

Any material suitable for antireflective coating may be used toconstruct the partial mirror. Exemplary materials include metals such asaluminum, silver, or gold or compounds such as magnesium fluoride.Additional exemplary coated materials are sold under the trademarkHEBBAR by CVI Melles Griot (Albuquerque, N. Mex.).

Coatings of the desired thickness can be fabricated by any method knownin the art including, for example, vacuum deposition, electricbombardment vaporization, plasma ion-assisted deposition (PIAD), carbonvapor deposition, plasma vapor deposition, and related techniques. Invacuum deposition, a substrate is put in a vacuum chamber along with ametal crucible holding the coating substance. A high current (e.g., 100A) is passed through the coating material, vaporizing it. Due to thevacuum, the vaporized material disperses to the material to be coated.

In certain aspects, the filter and gain chip assembly are connected withoptical fibers, such that light transmitted from the filter travelsthrough the optical fiber to the gain chip assembly and from the gainchip assembly back into the filter. In certain aspects of the invention,one optical fiber transmits light from the filter to the gain chipassembly, while a second optical fiber transmits light from the gainchip assembly back to the filter.

Optical fibers are flexible, transparent fibers able to transmit lightfrom one end of the fiber to the other. Optical fibers can be preparedfrom glass (silica) or from a variety of plastic polymers. Opticalfibers usually include a transparent core surrounded by a transparentcladding material with a lower index of refraction. Light is kept in thecore by total internal reflection, which causes the fiber to act as awaveguide or “light pipe.”

Any type of optical fiber is useful for practicing the invention,including multi-mode fibers (MMF) and single-mode fibers (SMF). MMFfibers support many propogation paths or transverse modes while SMFfibers support only a single mode. MMF fibers usually have wider corediameter than SMF fibers, and are used for short-distance communicationlinks and for applications where high power must be transmitted. SMFfibers are often used for communication links longer than 1000 meters.Assemblies of multiple fibers can also be prepared in wrapped bundles,which are often used for imaging procedures. The selection of theappropriate fibers and their connection to the various componentsdescribed herein is within the skill of the art. For example, MMF fibershaving a wavelength range of 400-2400 nm are available from Thorlabs(Item No. AFS50/125Y).

Further embodiments of the invention include an optical circulator forredirecting light among the various components encompassed by theinvention. For example, light can travel from the filter to thecirculator through an optical fiber connecting both the filter and thecirculator. The light can then travel from the circulator to the gainchip assembly and back again via a separate optical fiber connecting thecirculator to the gain chip assembly.

An optical circulator is a special fiber-optic component that can beused to separate optical signals that travel in different directions inan optical fiber. Optical circulators are typically three-port devices,configured such that light entering any port exits from the next. Thismeans that if light enters port 1, it is emitted from port 2, but ifsome of the emitted light is reflected back to the circulator, it doesnot come out of port 1, but instead exits from port 3. In this sense,fiber optic circulators act as signal routers, transmitting light froman input fiber at a first port to an output fiber at a second port, butdirecting light that returns along the output fiber to a third port.Circulators protect the input fiber from return power, but also allowthe rejected light to be employed. The selection of the appropriatecirculators and their connection to the various components describedherein is within the skill of the art. For example, optical circulatorshaving a wavelength range of 1525-1610 nm are available from Thorlabs,Inc (Item No. 6015-3-APC).

As encompassed by the invention, light from the filter enters thecirculator at a first port, and exits the circulator at a second port,which is connected to the gain chip assembly. When the light amplifiedin the gain chip assembly reaches the lasing threshold, light is emittedfrom the gain chip assembly. The emitted light meets a partial mirroroutside the gain chip assembly, whereupon a portion of the amplifiedlight is reflected back into the assembly. The amplified light isamplified for a second time in the gain chip assembly. This re-amplifiedlight travels back to the circulator, entering the circulator at thesecond port. The re-amplified light then exits the circulator out thethird port, whereupon the re-amplified light travels back to the filterand the whole process can begin again.

The present invention can operate as a light source for a variety ofuses, including imaging applications. In certain aspects, theunreflected portion of the light leaving the gain chip assembly isdirected to an optical tomography (OCT) system. Systems and methods ofthe invention are particularly amenable for use in OCT as the providedsystems and methods can improve image quality and reduce the incidenceof parasitic lasing.

Measuring a phase change in one of two beams from a coherent light isemployed in optical coherence tomography. Commercially available OCTsystems are employed in diverse applications, including art conservationand diagnostic medicine, e.g., ophthalmology. Recently, it has alsobegun to be used in interventional cardiology to help diagnose coronaryheart disease. OCT systems and methods are described in U.S. PatentApplication Nos. 2011/0152771; 2010/0220334; 2009/0043191; 2008/0291463;and 2008/0180683, the contents of which are hereby incorporated byreference in their entirety.

Various lumen of biological structures may be imaged with theaforementioned imaging technologies in addition to blood vessels,including, but not limited to, vasculature of the lymphatic and nervoussystems, various structures of the gastrointestinal tract includinglumen of the small intestine, large intestine, stomach, esophagus,colon, pancreatic duct, bile duct, hepatic duct, lumen of thereproductive tract including the vas deferens, vagina, uterus, andfallopian tubes, structures of the urinary tract including urinarycollecting ducts, renal tubules, ureter, bladder, and structures of thehead, neck, and pulmonary system including sinuses, parotid, trachea,bronchi, and lungs.

In OCT, a light source delivers a beam of light to an imaging device toimage target tissue. Within the light source is an optical amplifier andan tunable filter that allows that allows a user to select a wavelengthof light to be amplified. Wavelengths commonly used in medicalapplications include near-infrared light, for example, 800 nm forshallow, high-resolution scans or 1700 nm for deep scans.

Generally, there are two types of OCT systems, common beam path systemsand differential beam path systems, which differ from each other basedupon the optical layout of the systems. A common beam path system sendsall produced light through a single optical fiber to generate areference signal and a sample signal, whereupon a differential beam pathsystem splits the produced light such that a portion of the light isdirected to the sample and the other portion is directed to a referencesurface. The reflected light from the sample is recombined with thesignal from the reference surface of detection. Common beam pathinterferometers are further described in, for example, U.S. Pat. Nos.7,999,938; 7,995,210; and 7,787,127, the contents of which areincorporated by reference herein in its entirety.

In a differential beam path system, amplified light from a light sourceis inputted into an interferometer with a portion of light directed to asample and the other portion directed to a reference surface. A distalend of an optical fiber is interfaced with a catheter for interrogationof the target tissue during a catheterization procedure. The reflectedlight from the tissue is recombined with the signal from the referencesurface, forming interference fringes that allow precise depth-resolvedimaging of the target tissue on a micron scale. Exemplary differentialbeam path interferometers are further described in, for example, U.S.Pat. Nos. 6,134,003; and 6,421,164, the contents of which areincorporated by reference herein in its entirety.

In certain embodiments, the invention can be used in conjunction with adifferential beam path OCT system with intravascular imaging capabilityas illustrated in FIG. 4. In these embodiments, systems and methods ofthe invention can be used to provide a light source of narrow wavelengthlight. For intravascular imaging, a light beam is delivered to thevessel lumen via a fiber-optic based imaging catheter 826. The imagingcatheter is connected through hardware to software on a hostworkstation. The hardware includes an imaging engine 859 and a handheldpatient interface module (PIM) 839 that includes user controls. Theproximal end of the imaging catheter is connected to PIM 839, which isconnected to an imaging engine as shown in FIG. 4.

As shown in FIG. 5, the imaging engine 859 (e.g., bedside unit) houses apower supply 849, a light source 827 in accordance with the methods andsystems described herein, interferometer 931, and variable delay line835 as well as a data acquisition (DAQ) board 855 and optical controllerboard (OCB) 854. A PIM cable 841 connects the imaging engine 859 to thePIM 839 and an engine cable 845 connects the imaging engine 859 to thehost workstation.

FIG. 6 shows the light path in an exemplary embodiment of the invention.Light for image capture originates within the light source 827. Thislight is split between an OCT interferometer 905 and an auxiliaryinterferometer 911. The OCT interferometer generates the OCT imagesignal and the auxiliary, or “clock” interferometer characterizes thewavelength tuning nonlinearity in the light source and generates adigitizer sample clock.

In certain embodiments, each interferometer is configured in aMach-Zehnder layout and uses single mode fiber optics to guide thelight. Fibers are connected via LC/APC connectors or protected fusionsplices. By controlling the split ratio between the OCT and auxiliaryinterferometers with splitter 901, the optical power in the auxiliaryinterferometer is controlled to optimize the signal in the auxiliaryinterferometer. Within the auxiliary interferometer, light is split andrecombined by a pair of 50/50 coupler/splitters.

Light directed to the main OCT interferometer is also split by splitter917 and recombined by splitter 919 with an asymmetric split ratio. Themajority of the light is guided into the sample path 913 and theremainder into a reference path 915. The sample path includes opticalfibers running through the PIM 839 and the imaging catheter 826 andterminating at the distal end of the imaging catheter 826 where theimage is captured.

Typical intravascular OCT involves introducing the imaging catheter intoa patients' target vessel using standard interventional techniques andtools such as a guidewire, guide catheter, and angiography system. Whenoperation is triggered from the PIM or control console, the imaging coreof the catheter rotates while collecting image data that it delivers tothe console screen. Rotation is driven by spin motor 861 whiletranslation is driven by pullback motor 865, as shown in FIG. 7. Bloodin the vessel is temporarily flushed with a clear solution while a motortranslates the catheter longitudinally through the vessel.

In certain embodiments, the imaging catheter has a crossing profile of2.4 F (0.8 mm) and transmits focused OCT imaging light to and from thevessel of interest. Embedded microprocessors running firmware in boththe PIM and the imaging engine control the system. The imaging catheterincludes a rotating and longitudinally-translating inner core containedwithin an outer sheath. Using light provided by the imaging engine, theinner core detects reflected light. This reflected light is thentransmitted along a sample path to be recombined with the light from thereference path.

A variable delay line (VDL) 925 on the reference path uses an adjustablefiber coil to match the length of the reference path 915 to the lengthof the sample path 913. The reference path length is adjusted bytranslating a mirror on a lead screw based translation stage that isactuated electromechanically by a small stepper motor. The free-spaceoptical beam on the inside of the VDL 925 experiences more delay as themirror moves away from the fixed input/output fiber. Stepper movement isunder firmware/software control.

Light from the reference path is combined with light from the samplepath. This light is split into orthogonal polarization states, resultingin RF-band polarization-diverse temporal interference fringe signals.The interference fringe signals are converted to photocurrents using PIMphotodiodes 929 a and 929 b on the OCG as shown in FIG. 6. Theinterfering, polarization splitting, and detection steps are performedby a polarization diversity module (PDM) on the OCB. Signal from the OCBis sent to the DAQ 855, shown in FIG. 5. The DAQ includes a digitalsignal processing (DSP) microprocessor and a field programmable gatearray (FPGA) to digitize signals and communicate with the host workstation and the PIM. The FPGA converts raw optical signals intomeaningful OCT images. The DAQ also compresses data as necessary toreduce image transfer bandwidth to 1 Gbps.

In certain embodiments, the invention provides a light source for OCTincluding a filter, a gain chip assembly comprising a gain medium, and apartially reflecting mirror. The components are configured such thatlight passes from the filter to the gain chip assembly and a portion ofthe light is reflected by the partial mirror back through the gain chipassembly before the light passes again through the filter.

Any filter known in the art compatible with the invention may be used,including, for example, a tunable filter. The filter is included todeliver light of a specified wavelength into an optical amplifier. Thefilter typically has a peak reflectivity and a background reflectivity.In some embodiments, the system includes a commercial off-the-shelf(COTS) filter. One exemplary filter for use with the use with theinvention is filter module TFM-687 by Axsun technologies, Inc.(Billerica, Mass.). An exemplary tunable optical filter exhibits 20%reflectivity and 29 dB contrast ratio.

Any optical amplifier or laser known in the art and compatible with theinvention may be used as the gain component including, for example, asemiconductor optical amplifier. The amplifier amplifies the light to asufficient output power for imaging by OCT. The amplifier typically hasa semiconductor gain medium and an optical cavity. In some embodiments,a system includes a COTS amplifier. One exemplary optical amplifier foruse with the invention is booster optical amplifier serial numberBOA1130S, BOA1130P, or BOA-8702-11820.4.B01 from Thorlabs (Newton,N.J.). An exemplary optical amplifier has a center wavelength of 1285 nmand a small signal gain of 30 dB with a chip length of 1.5 mm.

A mirror can be coated with wavelength dependent material. Suitablematerials are available from Unioriental Optics co., Ltd (Zhong Guan CunScience Park, Beijing, China).

In certain embodiments, the invention provides a light source that emitsnarrow wavelength light for OCT systems like those shown in FIGS. 4 and5. Exemplary components of light source 827 are illustratedschematically in FIG. 3. Tunable optical filter 801 provides light to anoptical circulator 802 at a first port 802 a via a first optical fiber803. The light exits the circulator 802 at a second port 802 b andtravels through a second optical fiber 804 to a gain chip assembly 805.The operation of the gain chip assembly 805 has already been presentedschematically in FIG. 2. When the light amplified in the gain chipassembly 805 reaches the lasing threshold, light is emitted from theassembly 805. The emitted light meets a partial mirror 806 outside thegain chip assembly 805, whereupon a portion of the light is reflectedback into the assembly 805. The amplified light is amplified for asecond time in the gain chip assembly 805. This re-amplified lighttravels back to the circulator 802 through the second optical fiber 804,entering the circulator 802 at the second port 802 b. The re-amplifiedlight then exits the circulator 802 out the third port 802 c, whereuponthe re-amplified light travels back to the filter 801 via a thirdoptical fiber 807 and the whole process can begin again. Accordingly,systems and methods are able to produce narrow emissions of light fasterthan conventional methods by transmitting light through a filter andpassing a portion of the filtered light through the gain chip assemblyat least two times before that portion of filtered light passes againthrough the filter. This reduction in time to achieve the desiredwavelength results in improved image quality and reduces the incidenceof parasitic lasing.

INCORPORATION BY REFERENCE

References and citations to other documents, such as patents, patentapplications, patent publications, journals, books, papers, webcontents, have been made throughout this disclosure. All such documentsare hereby incorporated herein by reference in their entirety for allpurposes.

EQUIVALENTS

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The foregoingembodiments are therefore to be considered in all respects illustrativerather than limiting on the invention described herein. Scope of theinvention is thus indicated by the appended claims rather than by theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are therefore intended to be embracedtherein.

What is claimed is:
 1. A method of narrowing a wavelength emission oflight, the method comprising: transmitting light through a filter in afirst direction; sending the light from the filter through an opticalcirculator to a gain medium in a gain chip assembly a first time;reflecting a portion of the light leaving the gain chip assembly backthrough the gain medium in the gain chip assembly a second time and backinto the optical circulator; sending the reflected portion of the lightfrom the gain chip assembly back through the filter again in the firstdirection; sending the reflected, filtered portion of the light throughthe optical circulator to the gain medium in the gain chip assembly fora third time; directing an unreflected portion of the light leaving thegain chip assembly, after the first time and after the third time, to anoptical coherence tomography (OCT) system with intravascular imagingcapability, such that the OCT system receives light that has passedtwice through the filter and three times through the gain chip assembly.2. The method of claim 1, wherein reflecting is accomplished using apartial mirror.
 3. The method according to claim 1, wherein the filteris a tunable filter.
 4. The method of claim 3, wherein the filter is avoltage-controlled optical attenuator.
 5. The method of claim 1, whereinthe gain medium is a semiconductor.
 6. A system for narrowing awavelength emission of light, the system comprising: a filter; anoptical circulator; a gain chip assembly comprising a gain medium, apartially reflecting mirror; the system being configured such that lightpasses from the filter in a first direction, through the opticalcirculator, to the gain chip assembly and a portion of the light isreflected by the partial mirror back through the gain chip assembly andback through the optical circulator where the light is directed throughthe filter again in the first direction, through the optical circulator,and back through the gain chip assembly and to the partial mirror againsuch that light reflected by the partial mirror is filtered once andpasses through the gain chip assembly twice each time before reachingthe partial mirror again; an optical coherence tomography (OCT) systemwith intravascular imaging capability wherein the OCT system isconfigured to receive an unreflected portion of the light that passesthrough the gain chip assembly.
 7. The system according to claim 6,wherein the filter is a tunable filter.
 8. The system of claim 7,wherein the filter is a voltage-controlled optical attenuator.
 9. Thesystem of claim 8, wherein the gain medium is a semiconductor.